To meet industry needs for electrical power converters which convert AC to DC without injecting large amounts of harmonic currents into the power system, numerous transformer and converter topologies are available, some of which are described in the documents referenced above. Methods whereby three-phase power is converted to nine-phase power before processing, are described in several U.S. patents. For many industrial applications a small step down of voltage is desirable. Cost effective designs used in the variable frequency drive industry, and cited in the references above, use at least six windings on each phase of an autotransformer. A polygon transformer with five windings per phase, such as described by Zou et. al. requires only five windings per phase, but has the disadvantage of having a large circulating third harmonic current which significantly increases the transformer kVA rating.
The invention described here provides for an efficient design that in its preferred embodiment using only five windings per phase. It is a simplification and improvement on the methods described in the referenced patents, namely U.S. Pat. Nos. 5,124,904, 5,619,407, and 6,335,872.